A fatigue and creep study in austenitic stainless steel 316L used in exhaust pipes of naval gas turbines

Exhaust pipes of naval gas turbines are made of thin wall tubing of stainless steel grade AISI 316L. The tubes are fabricated by butt welding of different sections with longitudinal and circumferential joints. The plate thickness is about 4.0 mm, and the working temperature varies between 600 °C and 400 °C in the critical zones of the pipe, in the lower and central areas, respectively. The loadings in the tube induce high‐temperature fatigue and creep‐fatigue cracks have nucleated and propagated in the tube near some welded joints. The paper presents FCGR data obtained in CT specimens of used material taken from the pipes and tested at RT, 335, 500 and 600 °C. Preliminary creep data obtained in tension, in thin‐sheet specimens (about 4 mm thickness), also taken from the conduct wall and tested at 500, 550 and 600 °C are also given. These results are crucial to perform a fatigue‐creep interaction life assessment of the critical parts of the structure in the near future. Finally, the paper presents results of research work to investigate carbide precipitation and formation in virgin thin‐sheet specimens subjected to several types of thermal exposures. In some cases, 3–4 d was the time interval between exposures. Grain size measurements were carried out together with microstructural observations in the SEM. The influence of time, temperature and time interval between thermal exposures was assessed comparing the microstructures.     

Failure analysis in 316L stainless steel supracondylar blade plate

A 316L stainless steel blade plate implant used for fixation of a femoral fracture in a female patient failed catastrophically at four months of service. The failure examination included visual inspection, chemical analysis, metallography, hardness testing, as well as macroscopic observations using scanning electron microscope with EDS (energy-dispersive X-ray spectroscopy). The visual inspection and X-ray radiography images exhibited that there was heterogeneous bone callus formation and the fixation screws showed surface damage by fretting wear. On the other hand, SEM results showed evidence of fatigue based on the characteristics of the fracture surfaces. It was observed that the particular high curvature femur of the patient resulted in high tensile stress between bone and screws threads. It promoted rapid losing of primary fixation and subsequently generated a high stress concentration in the plate at cantilever which resulted in premature failure by fatigue.     

Failure analysis of a T-joint of AISI type 316 L stainless steel

The flow of sodium through the various circuits of a PFBR is controlled by centrifugal pumps. T-joints, made of type 316 LN stainless steel, will be used as pipe fittings to connect sodium pumps in the secondary side of the PFBR. However, two such T-joints received from the manufacturer were found to have developed fine cracks on the surface, which were seen after the pickling and passivation operation was carried out at the user's site. Visual, metallographic and fractographic examinations suggested that the failure occurred due to fatigue, which initiated because of surface roughening caused by the fabrication process. Initial large grain size of the material caused surface roughening to occur during fabrication. The rough surface led to initiation of fatigue cracks during fabrication, which was not carried out in one stretch as recommended by the user. Absence of corrosion product on the surface suggested no role of corrosion in the failure. The high hardness of the material indicated non-compliance with the user's specifications by the manufacturer vis-à-vis post-fabrication annealing. To avoid future failures, it was recommended that proper selection of material with respect to starting grain size should be made; qualified fabrication procedures should be adopted and followed so as to avoid cyclic loading on the component during fabrication; and appropriate solution annealing at 1323 K should be carried out to relieve cold work.     

Stress corrosion cracking in 316L stainless steel bellows of a pressure safety valve

There were three consecutive occurrences of bellows failure in a particular pressure safety valve (PSV) of a petroleum refinery within a time span of one week. The bellows were made of 316L grade austenitic stainless steel, and the PSV was mounted on one of the vessels of vacuum gas oil service in a hydrocracker unit. Metallurgical analysis of the failed bellows revealed that the failure had occurred by stress corrosion cracking (SCC). It was found that the SCC was promoted in the bellows due to the presence of high amount of chloride ions in the operating environment. Studies confirmed that SCC had initiated from the outer surface of the bellows and propagated inwards, resulting in leakage of hydrocarbon from the PSV. The source of chlorine in the environment was identified. It was discovered that SCC in the bellows was caused due to a previous failure in the heavy polynuclear aromatics (HPNA) absorption bed located upstream the process flow line. This failure was due to the presence of high concentrations of chlorine in the granulated activated carbon that was used in the HPNA absorption bed. During the repair of the HPNA bed, there was deposition of carbon soot on the body of the PSV. This carbon soot was the source of chloride ions for SCC to occur in the bellows. Generally, in chloride SCC, crack propagation in 316L SS takes place by transgranular mode. In the present case, however, the crack propagation was predominantly by intergranular mode. The metallurgical factors responsible for this change in micro-mechanism of crack propagation during SCC have been discussed.     

Microstructural creep damage in welded joints of 316L stainless steel

The present work has been undertaken to study creep damage in welded joints. The complex dual phase microstructure of 316L welds are simulated by manually filling a mould with longitudinally deposited weld beads. Most of the moulded specimens were then aged for 2000 hours at 600°C. High resolution scanning electron microscopy was extensively used to examine the microstructure of the welded material before and after ageing. Columnar grains of austenite constitute a matrix in which thin dendrites of δ-ferrite can be found. The ageing generates the precipitation of carbides, resulting in less transformation in the material. Smooth and notched creep specimens were cut from the mould and tested at 600°C under different stress levels. The creep life of the simulated welded material is shown to be lower than that of the base material. Microstructural observations reveal that creep cavities are preferentially located along the austenite grain boundaries. This analysis of intergranular damage on test specimens is conducted to obtain a predictive damage law which could be used to calculate the lifetime of welded joints.     

Hall-Petch behaviour of 316L austenitic stainless steel at elevated temperatures

Abstract

The plastic deformation behaviour of two different batches (having differences in chemical composition) of 316L austenitic stainless steel has been explored in the 200-800°C temperature range as a function of grain size. The plastic behaviour is correlated with microstructural observations of annealed and deformed samples. The microstructural parameters measured in this study are grain size, grain size and shape distribution, grain aspect ratio, and the distribution of dihedral angles. Hardness measurements were also performed to assess the hardness profile across the grains. The applicability of Hall-Petch relationship was tested in the 200-800°C temperature range. It is observed that the Hall-Petch relationship is applicable in the coarse grain regime (d≥6 μm) and Kocks composite relationship (σ versus d^-1) in the fine grain regime (d≤6 μm) of batch 1 samples in the 200-600^°C temperature range. At 800°C, the Hall-Petch data is widely scattered and the scatter increases with increasing strain. The variation of Hall-Petch parameters and Kocks parameters with strain and temperature are analysed on the basis of changes in the microstructural parameters. The operating deformation mechanisms in different temperature and strain ranges are discussed on the basis of variation of microstructural parameters with strain and temperature.     

Swelling of 316L austenitic stainless steel induced by plasma nitriding

Swelling of 316L austenitic stainless steel plasma nitrided at 400°C under floating potential has been investigated using electron back scattered diffraction and white-light interferometry. Swelling of individual grains strongly depends on their crystallographic orientation, similarly to the thickness of the nitrided layer. After 1 h of treatment, swelling is maximum for the 〈001〉 oriented grains and minimum for the 〈111〉 oriented grains. After 8 and 33 h of nitriding, the maximum of swelling is observed in the grains having their normal direction at about 15° from the 〈001〉 orientation. These results are discussed on the basis of plastic strain after comparison with calculated swellings of the 〈001〉 and 〈111〉 oriented grains, using the thickness of the nitrided layer deduced from the trapping–detrapping diffusion model and a rough estimation of the plastic strain. The satisfactory agreement between experimental and calculated swellings supports the idea that swelling results from the lattice expansion due to the incorporation of nitrogen plus an elastic strain and a plastic strain. For individual grains of the 316L matrix, nitriding leads to a tensile-like elongation of high magnitude (around 20%) and it might be the origin of the lattice rotations which were previously observed after nitriding.     

Tensile ductility and deformation mechanisms of a nanotwinned 316L austenitic stainless steel

A nanotwinned 316 L austenitic stainless steel was prepared by means of surface mechanical grinding treatment.After recovery annealing,the density of dislocations decreases obviously while the average twin/matrix lamella thickness still keeps in the nanometer scale.The annealed nanotwinned sample exhibits a high tensile yield strength of 771 MPa and a considerate uniform elongation of 8%.TEM observations showed that accommodating more dislocations and secondary twinning inside the nanotwins contribute to the enhanced ductility and work hardening rate of the annealed nanotwinned sample.     

Thermal shock testing of type 316 stainless steel using sodium

The growth of cracks from spark-machined slits and gouged grooves under thermal shock conditions has been studied in 316 stainless steel using sodium as the heat transfer medium. The cycles consisted of a step increase in temperature followed by a tensile hold at 600°C. Comparative uniaxial creep-fatigue tests have been performed isothermally at 600°C. Differences in the stress distributions in the uniaxial and thermal shock specimens led to higher crack growth rates in the uniaxial tests. These have been taken into account in theoretical predictions based on the creep parameter C*. This led to very reasonable upper bounds to the experimentally observed increase in crack growth rate due to hold time for both types of specimen.     

Notch fatigue behaviour of low-temperature gaseous carburised 316L austenitic stainless steel

ABSTRACT

The influence of low-temperature gaseous carburisation on notch fatigue behaviour of 316L steel under cyclic axial loading was investigated. After carburisation, the carburised case was well distributed at the surface region and was not influenced by the notch geometry. Low-temperature carburisation considerably enhanced the notch fatigue performance, which led to 32% and 44% increase in the endurance limits for the specimens with stress concentration factors K _t?=?1.91 and 3.91, respectively. The notch sensitivity of 316L steel reduced after carburisation. Irrespective of the applied stress amplitude, the fatigue crack nucleation sites were always at the notch root surface for the untreated specimens. For the carburised specimens, fatigue cracks nucleation changed from surface at high-level stress to subsurface at low-level stress.     

316L奥氏体不锈钢中时效条件下析出相演变行为的研究

通过定量金相,SEM&EDS、TEM等实验技术分析316L奥氏体不锈钢中析出相随时效时间、温度的变化,并测定析出相的体积分数与尺寸.结合热力学计算表明:在316L奥氏体不锈钢中,经850℃时效处理后,析出相为M23C6型碳化物,且随着时效时间的延长,析出量明显增多,尺寸增大;经650℃时效处理100 h后,主要析出相类型为χ相.     

Irradiation behavior of nanostructured 316 austenitic stainless steel

In order to get information about radiation resistance of ultrafine grained austenitic stainless steels, a 316 steel was deformed by high pressure torsion. The mean diameter of the grain after deformation was 40 nm. This material was annealed at 350 °C for 24 h or irradiated with 160 keV iron ions at 350 °C. Changes in the microstructure during annealing or irradiation were characterised by transmission electron microscopy (grain size) and laser assisted tomographic atom probe (solute distribution). Results indicate that this annealing has no influence on the grain size whereas the grain diameter increases under irradiation. Concerning the solute distribution, atom probe investigations show evidence of radiation-induced segregation at grain boundaries. Indeed, after irradiation, grain boundaries are enriched in nickel and silicon and depleted in chromium. On the contrary, no intragranular extended defects or precipitation are observed after irradiation.     

Effect of thermo-mechanical processing on texture evolution in austenitic stainless steel 316L

In this study we implemented two rolling modes to investigate the strain path effect on deformation and annealing textures of austenitic stainless steel 316L. We applied unidirectional rolling and cross-rolling to achieve up to a 90% reduction in thickness. Results show that for deformed austenite, Brass, Goss and γ-fibre were the main texture components in the unidirectional rolled sample, while Brass was the dominant texture in the cross-rolled sample. In addition, rotated Copper and rotated Cube were the main textures of martensite after 90% reduction for unidirectional rolled and cross-rolled samples, respectively. Results also show that recrystallization texture has a direct correlation to that of deformed austenite since transformed martensite reversion was athermal. After recrystallization, Brass and a combination of Brass and Goss were the dominant textures for cross-rolled and unidirectional rolled samples, respectively.     

Failure analysis of 316L stainless steel tubing of the high-pressure still condenser

Sherritt International Corporation experienced corrosion failures with the 316L stainless steel tubing in a high-pressure still condenser employed for ammonia recovery. A detailed failure analysis was conducted on the condenser tubing to determine the mode and the root cause of the failure. The analysis included both optical and scanning electron microscopy (SEM) of the inner and outer surfaces of the tube as well as characterization of the corrosion products using energy-dispersive X-ray spectroscopy (EDX). Results revealed that the corrosion attack was confined to the first ～100 mm of the tubing at the inlet where the tube was connected to the top tubesheet. The tube suffered both external stress-corrosion cracking (SCC) and crevice corrosion from the shell side (water side), and wall thinning of the inner surface (the tube side) due to erosion corrosion. It was evident that failure of one of the tubes occurred due to SCC that penetrated the whole wall thickness and resulted in a leak failure. Some prevention measures are proposed to avoid this type of corrosion attack in the future.     

The effect of pre-strain on tensile behaviour of 316L austenitic stainless steel

ABSTRACT

The effect of pre-strain on tensile behaviour of 316L austenitic stainless steel was investigated, focusing on strain rate sensitivity, temperature sensitivity and strain hardening. Experimental data showed that strain rate sensitivity, temperature sensitivity and strain hardening were weakening with pre-strain. Meanwhile, the variation of microstructure with pre-strain was observed by optical microscopy, scanning electronic microscopy and X-ray diffraction. Then, the mechanical properties of pre-strained material were correlated with the increase in dislocation density and mechanical twinning with pre-strain. Finally, an improved Arrhenius-PS model considering the effect of pre-strain was developed.

This paper is part of a thematic issue on Nuclear Materials.     

Influence of crystallographic textures on tensile properties of 316L austenitic stainless steel

Role of cold rolling texture on the tensile properties of the cold rolled and cold rolled and annealed AISI 316L austenitic stainless steel is described here. The solution-annealed stainless steel plates were unidirectionally cold rolled to 50, 70 and 90% of reduction in thickness. The cold rolled material was annealed at 500–900 °C annealing temperatures. X-ray diffraction technique was employed to study the texture evolution in cold rolled as well as cold rolled and annealed conditions. The texture components that evolved were translated into slip transmission number ‘λ’ and Schmid factor ‘μ’. These two parameters were correlated with the tensile properties of the material. The tensile properties were evaluated under all processing conditions. Softening of the cold rolled material was observed after annealing with increasing annealing temperatures. From the stress–strain curves, strain hardening coefficient ‘n’ and strain hardening rate ‘θ’ were determined. It was found that the effect of texture on tensile behaviour could be understood clearly by strain hardening rate. Out of the two parameters, ‘n’ and ‘θ’, strain hardening rate was found to be more sensitive to type of texture in the material.     

Low-temperature carburised AISI 316L austenitic stainless steel: Wear and corrosion behaviour

Low temperature carburising (LTC) is a thermochemical treatment designed so as to achieve a good combination of wear and corrosion resistance in stainless and duplex steels. In this work, the influence of LTC on both corrosion and dry sliding behaviour of AISI 316L was investigated. LTC significantly enhanced surface hardness, due to the formation of the carbon-supersaturated S-phase. Consequently, the wear behaviour (evaluated against different countermaterials) improved, due to increased resistance to plastic deformation, as well as to decreased tendency towards adhesion. In order to evaluate the corrosion behaviour, electrochemical measurements were performed both in conventional environments and in reference conditions for the food industry. The results showed a significantly improved corrosion resistance in chloride environments, where the formation of a C-rich surface layer ennobles the treated steel, even though pitting corrosion was observed at very high anodic potentials. Conversely, the treated steel showed comparable (in acetic acid) or worse (in a sanitising solution) behaviour than the untreated one. In sulphuric acid the treated steel did not passivate, but it corroded at a limiting current density much lower that the critical current density for AISI 316L passivation.